Date: Fri, 14 Aug 2015 04:45:34 -0700 (MST)
From: zhangjinyu <beijing_pg@163.com>
To: pgsql-bugs@postgresql.org
Message-ID: <1439552734803-5862147.post@n5.nabble.com>
In-Reply-To: 
 <CAJrrPGd--dTUoRpwiSmNVpa56sbCMMB-xB4=Qc4um1NNnt4uhQ@mail.gmail.com>
References: <20150807121643.32633.60549@wrigleys.postgresql.org>
 <CAJrrPGd--dTUoRpwiSmNVpa56sbCMMB-xB4=Qc4um1NNnt4uhQ@mail.gmail.com>
Subject: Re: BUG #13541: There is a visibility issue when run some DDL and
 Query. The time window is very shot
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The test cases is not clear in last mail,  the following shows all test
cases.
1. Table precondition  
create table t(c1 int);
create table t2(c1 int):
insert into t values(1),(2),(3);
select xmin from t ;   (xmin=10043)
2. Step1: 
   session1:  begin transaction isolation level repeatable read;
                 select * from t2;   (query on other table only for get
snapshot)
   Step2:
  session2: alter table t alter c1 set data type char(10);
               select xmin from t;   (new xid)
   Step3:
  session1:  select xmin from t;       (you can see 0 row, because the
snapshot is the same as the first query when transaction isolation level is
repeatable read.)

The DDL query(alter ...set data type ) rewrites all tuples with new
transaction xid,  so if the transaction isolation level is repeatable read,
that will cause wrong result.   Which DDL query rewrite all tuples with new
xid?
                                                                

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